1. Field of the Invention
The present invention relates generally to an apparatus for grinding end faces of ferrules for optical fiber connectors used for an optical fiber communication system. More particularly, the present invention relates to an improvement of an apparatus of the foregoing type for grinding end faces of optical fibers not only without angular error of each ground surface relative to a center axis of the corresponding optical fiber which arises with a conventional optical fiber end face grinding apparatus, but also without fluctuation of a quantity of each ground end face of the optical fiber.
2. Description of the Related Art
An end face of an optical fiber to be used for an optical fiber connector is usually ground so as to reduce connection loss. It is well known by the ordinary person skilled in the art that tile surfaces at the edges of the ferrule body and optical fiber can be made spherical by simultaneously polishing these surfaces. The spherical surface at the edges of the ferrule body and optical fiber can be obtained by the inclined spherical polishing of the surface on an elastic material disk when the surface is inclined with respect to the elastic material disk because of their circular deflection.
The inventor of the present invention has made several proposals in connection with a grinding operation for end faces of optical fibers. For example, he invented a fundamental technical concept related to a connector of which one end face is slantwise ground with a certain angle (hereinafter referred to simply as an APC representing an angle polish connector) in association with his prior invention (U.S. Pat. No. 5,140,660) of ferrules for optical fiber connectors. In addition, he invented an optical fiber grinding apparatus (U.S. Pat. Nos. 4,831,784 and 4,979,334) in which a technical concept related to an apparatus for simultaneously grinding end faces of a plurality of optical fibers is disclosed.
The present invention has been made to substantially improve the performances of the conventional grinding apparatuses mentioned above. To facilitate understanding of the present invention, the structure and inherent problems of the conventional grinding apparatus will be described below with reference to FIG. 1 to FIG. 3.
FIG. 1 is a partially exploded side view of a conventional apparatus for grinding end faces of a plurality of ferrules each having an optical fiber firmly received therein, schematically illustrating a fundamental structure of the conventional apparatus, and FIG. 2 and FIG. 3 are fragmentary vertical sectional views of the conventional apparatus shown in FIG. 1, particularly illustrating problems inherent in the conventional apparatuses.
As shown in the drawings, the grinding apparatus includes a platform 1 on which a rotary disc 2 and a driving mechanism (not shown) for rotationally driving the rotary disc 2 are mounted.
A circular grinding board 3 is integrally placed on the rotary disc 2 so as to allow them to be rotated together. For example, a rotary disc consisting of a plate-shaped elastic member and an abrasive paper adhesively placed on the elastic member as disclosed in the prior invention (U.S. Pat. No. 5,140,660) relating to a ferrule for an optical fiber connector can be noted as a typical grinding board employable for the grinding board 3.
A plurality of adapters 6 for firmly securing to a circular disc-shaped ferrule holder 4 a plurality of ferrules 5 each having an optical fiber 9 firmly received therein are arranged on the ferrule holder 4 in an equally spaced relationship as seen in the circumferential direction, and a cylindrical portion 8 having a hole 7 formed therein is integrated with the ferrule holder 4 at the central part of the rotary disc 2.
Each ferrule 5 having an optical fiber 9 firmly received therein is fitted into the corresponding adaptor 6 so that it is fixedly secured to the ferrule holder 4 by threadably tightening a cap nut 10.
An arm-shaped support rod holding member 11 is integrated with the platform 1 for the grinding apparatus, and a thrusting support rod 12 having a load W (not shown) applied thereto is inserted through a hole 13 formed at the foremost end of the holding member 11.
When the thrusting support rod 12 is inserted through the hole 7 of the cylindrical portion 8 at the central position of the rotary disc 2, it is thrusted in the direction toward the working surface of the grinding board 3.
As shown In FIG. 2, each ferrule 5 is firmly secured to the corresponding adapter 6 while projecting downward of the lower surface of the ferrule holder 4 by a distance H of 0.2 to 1.0 mm as measured from a reference plane of the adaptor 6 with which a flange of tile ferrule 5 comes in contact, and the foremost end of the ferrule 5 is thrusted against the working surface of the grinding board 3. While the foregoing state is maintained, the foremost end of the ferrule 5 is ground as the grinding board 3 is rotated.
However, with the conventional grinding apparatus constructed in the above-described manner, there often arises a dimensional error .DELTA.H of 0.05 to 0.2 mm based on the foregoing distance H attributable to an error derived from production of the ferrules 5 arranged on the ferrule holder 4 in that way.
In this case, the grinding load W is concentratively applied to one of the ferrules 5, i.e., a ferrule having a maximum quantity of projection from the ferrule holder 4, causing the ferrule holder 4 to be inclined as shown in FIG. 2. Thus, there is a possibility that the foremost ends of the ferrules 5 are ground while the foregoing inclined state is maintained. In other words, the end faces of the ferrules 5 are unavoidably ground while the parallel state of the lower surface of the ferrule holder 4 relative to the grinding board 3 is incorrectly maintained.
In addition, with the conventional grinding apparatus, there is recognized an undesirable phenomenon that as the grinding board 3 is rotated, the ferrule holder 4 is inclined in the forward direction relative to the rotating direction of the grinding board 3 represented by an arrow A due to the turning moment appearing on the ferrule holder 4 represented by arrows B and B', as shown in FIG. 3.
To prevent the ferrule holder 4 from being inclined in that way, it is necessary that the outer diameter of the thrusting support rod 12, the inner diameter of the hole 7 for receiving the thrusting support rod 12 and the inner diameter of the hole 13 formed through the thrusting rod holding member 11 are finished with higher fitting accuracy. Although careful attention is paid to rigidity of the respective components at the time of designing in order to eliminate or minimize deflection of the thrusting rod holding member 11 and others, the aforementioned problems are left still unsolved because the fitting clearance can not be reduced to a level of zero.
As is apparent from the above description, as far as the conventional grinding apparatus is constructed in the above-described manner, it is very difficult to completely eliminate an angular error relative to the angle defined by the optical axis of each optical fiber and the ground end face of each ferrule due to malfunctions such as fluctuation of the length of each ferrule 5, incorrect positional fitting of the ferrules 5 into the ferrule holder 4 or the like.
In the case of an APC which requires that each ferrule is slantwise ground with an angle of 8 to 12 degrees relative to the optical axis of each optical fiber so as to reduce return reflection loss, there is a possibility that the angular error arising on tile ground face of each ferrule 5 is undesirably amplified due to a large amount of grinding operation to be performed for the end face of the ferrule 5.